Refrigeration System

ABSTRACT

A refrigerant circuit ( 10 ) operates in a refrigeration cycle in which the pressure of refrigerant discharged from a compressor ( 22 ) is at or above the critical pressure. In performing an operation in which a first indoor heat exchanger ( 33   a ) performs a heating operation and, concurrently, a second indoor heat exchanger is made inactive, an indoor expansion valve ( 34   b ) associated with the inactive indoor heat exchanger ( 33   b ) is fully closed.

TECHNICAL FIELD

This invention relates to refrigeration systems in which each of aplurality of utilization side heat exchangers can individually perform aheating operation and particularly relates to measures againstrefrigerant liquefaction in inactive ones of the utilization side heatexchangers.

BACKGROUND ART

Refrigeration systems operating in a refrigeration cycle by circulatingrefrigerant therethrough are widely applied, such as to air conditioningsystems. Such air conditioning systems include a so-called multi-typeair conditioning system in which a plurality of indoor units areconnected in parallel to an outdoor unit.

For example, an air conditioning system disclosed in Patent Document 1includes a single outdoor unit having a compressor and an outdoor heatexchanger (heat-source side heat exchanger) and two indoor units eachhaving an indoor heat exchanger (utilization side heat exchanger). Twobranch pipes, each connected to an associated one of the two indoor heatexchangers, are provided with their respective electric motor-operatedvalves in association with the respective indoor heat exchangers.

In the air conditioning system, each of the indoor units canindividually perform a heating operation by controlling the opening ofthe associated electric motor-operated valve. Specifically, for example,when the two indoor units concurrently perform a heating operation, boththe electric motor-operated valves are opened at a predetermined openingto positively feed refrigerant into both the indoor heat exchangers. Asa result, heat is released from refrigerant flowing through both theindoor heat exchangers to room air, thereby heating respective roomspaces. On the other hand, for example, when only one of the indoorunits performs a heating operation, the electric motor-operated valveassociated with the active indoor unit is opened but the electricmotor-operated valve associated with the deactivated indoor unit isclosed. As a result, refrigerant is fed only into the indoor heatexchanger in the active indoor unit and the refrigerant in this indoorheat exchanger releases heat to room air.

Patent Document 1: Published Japanese Patent Application No. H08-159590

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

When only one of the two indoor units is continuously operated asdescribed above, there may occur a phenomenon in which refrigerant inthe inactive indoor heat exchanger condenses and accumulates therein, ora so-called refrigerant liquefaction. If refrigerant is thus graduallyliquefied in the inactive indoor heat exchanger, the indoor heatexchanger being active (in heating operation) becomes deficient in theamount of refrigerant flowing therethrough, which deteriorates theheating capacity of the active indoor unit.

The present invention has been made in view of the foregoing point and,therefore, an object thereof is to prevent refrigerant liquefaction inthe inactive utilization side heat exchanger.

Means to Solve the Problems

A first aspect of the invention is directed to a refrigeration systemincluding a refrigerant circuit (10) formed so that a plurality ofutilization side circuits (31 a, 31 b) including their respectiveutilization side heat exchangers (33 a, 33 b) and electricmotor-operated valves (34 a, 34 b) associated with the respectiveutilization side heat exchangers (33 a, 33 b) are connected in parallelto a heat-source side circuit (21) including a compressor (22) and aheat-source side heat exchanger (23), each of the utilization side heatexchangers (33 a, 33 b) being capable of individually performing aheating operation to release heat from refrigerant in the utilizationside heat exchanger (33 a, 33 b). Furthermore, in the refrigerationsystem, the refrigerant circuit (10) is configured to operate in arefrigeration cycle in which the pressure of refrigerant discharged fromthe compressor (22) is at or above the critical pressure.

The refrigeration system according to the fist aspect of the inventioncan perform an operation in which all of the utilization side heatexchangers (33 a, 33 b) perform the heating operation (hereinafter,referred to as a full operation) and an operation in which one or someof the utilization side heat exchangers (33 b) halt the heatingoperation and, concurrently, the rest of the utilization side heatexchangers (33 a) perform the heating operation (hereinafter, referredto as a partial operation).

Specifically, the full operation can be achieved by opening each of theelectric motor-operated valves (34 a, 34 b) associated with theutilization side heat exchangers (33 a, 33 b) at a predeterminedopening. Thus, in the full operation, refrigerant discharged from thecompressor (22) flows through each of the utilization side heatexchangers (33 a, 33 b). Consequently, heat is released from refrigerantflowing through each of the utilization side heat exchangers (33 a, 33b), whereby each utilization side heat exchanger (33 a, 33 b) performs aheating operation. As a result, each utilization side heat exchanger (33a, 33 b) heats a room space, for example.

On the other hand, in the case of halting the heating operation of oneor more utilization side heat exchangers (33 b) out of the utilizationside heat exchangers (33 a, 33 b), the electric motor-operated valve (34b) associated with each utilization side heat exchanger (33 b) to beinactive is set to a minute opening or fully closed and, concurrently,the electric motor-operated valve (34 a) associated with eachutilization side heat exchanger (33 a) to perform a heating operation isopened at a predetermined opening. As a result, refrigerant flowssubstantially only through the utilization side heat exchangers (33 a)in heating operation and each inactive utilization side heat exchanger(33 b) does not perform a heating operation.

As the refrigeration system performs such a partial operation, owing toreduction in the opening of the electric motor-operated valve (34 b) ineach deactivated unit, refrigerant gradually accumulates in the inactiveutilization side heat exchanger (33 b). In this case, if therefrigeration system operated in a refrigeration cycle using refrigerantmade, such as of HFC, to bring the discharge pressure of the compressorto a subcritical pressure and the deactivation of the utilization sideheat exchanger (33 b) dropped the ambient temperature thereof,refrigerant in the inactive utilization side heat exchanger (33 b) wouldgradually condense. As a result, refrigerant would liquefy in theinactive utilization side heat exchanger (33 b), which causes a problemthat the utilization side heat exchangers (33 a) in heating operationfall short of the amount of refrigerant flowing therethrough.

In this aspect of the invention, to prevent such refrigerantliquefaction in each inactive utilization side heat exchanger (33 b),the pressure of refrigerant discharged from the compressor (22) is setat or above the critical pressure. In other words, the refrigerantcircuit (10) of the refrigeration system according to this aspect of theinvention operates in a refrigeration cycle in which refrigerant reachesor exceeds its critical pressure (a so-called supercritical cycle). As aresult, in the partial operation, refrigerant in a critical stateaccumulates in the inactive utilization side heat exchanger (33 b) and,therefore, the refrigerant does not condense in the utilization sideheat exchanger (33 b). Thus, as compared with the conventionalrefrigerant circuit operating in a refrigeration cycle using refrigerantmade, such as of HFC, refrigerant does not change its phase in eachinactive utilization side heat exchanger (33 b) in this aspect of theinvention, whereby the rate of refrigerant liquefaction in the inactiveutilization side heat exchanger (33 b) becomes low.

A second aspect of the invention is the refrigeration system accordingto the first aspect of the invention and further including a controlmeans (51) that, in performing an operation in which at least one saidutilization side heat exchanger (33 a) in heating operation and at leastone said inactive utilization side heat exchanger (33 b) coexist, fullycloses the electric motor-operated valve (34 b) associated with the atleast one inactive utilization side heat exchanger (33 b).

In the second aspect of the invention, in performing the above partialoperation, the control means (51) fully closes the electricmotor-operated valve (34 b) associated with each inactive utilizationside heat exchanger (33 b). As a result, refrigerant graduallyaccumulates in each inactive utilization side heat exchanger (33 b).However, in this aspect of the invention, the amount of refrigerantliquefied in the inactive utilization side heat exchanger (33 b) issignificantly reduced since the refrigeration system operates in asupercritical cycle as described above.

Furthermore, since the electric motor-operated valve (34 b) is thusfully closed, refrigerant flows only through the utilization side heatexchangers (33 a) in heating operation. Therefore, it can be avoidedthat refrigerant flows through each inactive utilization side heatexchanger (33 b) to cause wasteful heat release from the utilizationside heat exchanger (33 b).

A third aspect of the invention is the refrigeration system according tothe second aspect of the invention, wherein when a first specified timet1 has passed since full closure of the electric motor-operated valve(34 b) associated with the at least one inactive utilization side heatexchanger (33 b), the control means (51) temporarily opens the electricmotor-operated valve (34 b) for a second specified time t2.

In the third aspect of the invention, when in performing the partialoperation the first specified time t1 has passed since full closure ofthe electric motor-operated valve (34 b) associated with each inactiveutilization side heat exchanger (33 b), the control means (51) opens theelectric motor-operated valve (34 b) to a predetermined opening(preferably, a relatively minute opening). The reason for this is thatwhen the partial operation is continued for a long period of time,refrigerant might gradually liquefy in each inactive utilization sideheat exchanger (33 b) even when the refrigeration system operates in asupercritical cycle as described above. For this reason, in the partialoperation in this aspect of the invention, when the first specified timet1 has passed, the electric motor-operated valve (34 b) is forciblyopened so that refrigerant flows through the inactive utilization sideheat exchanger (33 b) only for the second specified time t2. Thus,refrigerant in the inactive utilization side heat exchanger (33 b) flowsfor the second specified time t2, whereby the temperature of theutilization side heat exchanger (33 b) and its ambient temperatureincrease to eliminate refrigerant liquefaction. Then, when the secondspecified time t2 has passed, the electric motor-operated valve (34 b)is fully closed again.

A fourth aspect of the invention is the refrigeration system accordingto the third aspect of the invention, wherein each of the utilizationside heat exchangers (33 a, 33 b) is placed in a room and configured torelease heat from refrigerant to a room air, room temperature sensors(44, 45) for detecting the temperatures of rooms associated with therespective utilization side heat exchangers (33 a, 33 b) are providedaround the respective utilization side heat exchangers (33 a, 33 b), andthe refrigeration system further includes a correction means (52) thatcorrects one or both of the first specified time t1 and the secondspecified time t2 based on the temperature detected by the roomtemperature sensor (45) associated with the at least one inactiveutilization side heat exchanger (33 b).

In the fourth aspect of the invention, the correction means (52)corrects one or both of the first specified time t1 and the secondspecified time t2 based on the room temperature detected by the roomtemperature sensor (45) around each inactive utilization side heatexchanger (33 b).

More specifically, for example, when the room temperature around aninactive utilization side heat exchanger (33 b) is high, refrigerant isless likely to liquefy in the inactive utilization side heat exchanger(33 b). Therefore, in such a case, the period of time during which theassociated electric motor-operated valve (34 b) is fully closed can beextended by making a correction to increase the first specified time t1or a correction to decrease the second specified time t2. As a result,it can be avoided that refrigerant wastefully releases heat in theinactive utilization side heat exchanger (33 b).

On the other hand, for example, when the room temperature around aninactive utilization side heat exchanger (33 b) is low, refrigerant islikely to liquefy in the inactive utilization side heat exchanger (33b). Therefore, in such a case, refrigerant liquefaction in theutilization side heat exchanger (33 b) can be avoided in advance bymaking a correction to decrease the first specified time t1 or acorrection to increase the second specified time t2.

In a fifth aspect of the invention, the refrigeration system furtherincludes refrigerant density detecting devices (40, 41, 42, 43) fordetecting the refrigerant densities in the associated utilization sideheat exchangers (33 a, 33 b), wherein when the refrigerant densitydetected by at least one said refrigerant density detecting device (40,41, 43) associated with the at least one inactive utilization side heatexchanger (33 b) exceeds a specified refrigerant density after fullclosure of the electric motor-operated valve (34 b) associated with theat least one inactive utilization side heat exchanger (33 b), thecontrol means (51) temporarily opens the electric motor-operated valve(34 b).

In the fifth aspect of the invention, in performing the partialoperation, the refrigerant density in each inactive utilization sideheat exchanger (33 b) is detected by the associated refrigerant densitydetecting device (40, 41, 43) after full closure of the electricmotor-operated valve (34 b) associated with the inactive utilizationside heat exchanger (33 b). In other words, the refrigerant detectingmeans (40, 41, 43) indirectly detects the amount of refrigerantaccumulated in the inactive utilization side heat exchanger (33 b) basedon the refrigerant density. Then, when the detected refrigerant densityexceeds a specified refrigerant density, the control means (51)considers a large amount of refrigerant to be accumulated in theinactive utilization side heat exchanger (33 b) and temporarily opensthe electric motor-operated valve (34 b). As a result, refrigerantliquefaction in the inactive utilization side heat exchanger (33 b) canbe avoided in advance.

A sixth aspect of the invention is the refrigeration system according toany one of the first to fifth aspects of the invention, wherein therefrigerant circuit (10) is filled with carbon dioxide as refrigerant.

In the sixth aspect of the invention, the refrigerant circuit (10)operates in a supercritical cycle using carbon dioxide.

A seventh aspect of the invention is the refrigeration system accordingto any one of the second to fifth aspects of the invention and furtherincluding supply openings through which air having passed through theassociated utilization side heat exchangers (33 a, 33 b) is let out andopening/closing mechanisms for opening and closing the associated supplyopenings, wherein each of the opening/closing mechanisms is configuredto open the supply opening of the associated utilization side heatexchanger (33 b) when in heating operation and close the supply openingof the associated utilization side heat exchanger (33 a) when inactive.

The refrigeration system according to the seventh aspect of theinvention is provided with a plurality of supply openings associatedwith their respective utilization side heat exchangers (33 a, 33 b).Furthermore, each supply opening is provided with an opening/closingmechanism for opening and closing the supply opening. In this case, inthe full operation, the opening/closing mechanisms for all the supplyopenings are put into an open position, whereby air heated by theutilization side heat exchangers (33 a, 33 b) is supplied into rooms orthe like through the supply openings. On the other hand, in the partialoperation, the opening/closing mechanism for the supply opening in eachutilization side heat exchanger (33 a) in heating operation is put intoan open position but the opening/closing mechanism for the supplyopening in each inactive utilization side heat exchanger (33 b) is putinto a closed position. As a result, in each inactive utilization sideheat exchanger (33 b), it can be prevented that heat of refrigeranttherein escapes through the supply opening to another space, such as aroom. Therefore, the drop in the ambient temperature of each inactiveutilization side heat exchanger (33 b) can be restrained, wherebyrefrigerant liquefaction in this utilization side heat exchanger (33 b)can be effectively avoided.

Effects of the Invention

In the present invention, the refrigeration system, in which each of aplurality of utilization side heat exchangers (33 a, 33 b) canindividually perform a heating operation, operates in a supercriticalcycle in which the pressure of refrigerant discharged from thecompressor (22) is at or above the critical pressure. Thus, even when inthe above-stated partial operation the electric motor-operated valve (34b) in each deactivated unit is opened at a minute opening or fullyclosed, refrigerant is less likely to liquefy in the inactiveutilization side heat exchanger (33 a, 33 b). Therefore, according tothe present invention, it can be eliminated that each utilization sideheat exchanger (33 a) in heating operation falls short of the amount ofrefrigerant flowing therethrough, thereby providing a sufficient heatingcapacity of the utilization side heat exchanger (33 a) in heatingoperation.

Particularly in the second aspect of the invention, the electricmotor-operated valve (34 b) in each deactivated unit is fully closed inperforming the partial operation. Thus, according to the second aspectof the invention, all the refrigerant is fed to the utilization sideheat exchangers (33 a) in heating operation, whereby it can be avoidedthat each inactive utilization side heat exchanger (33 b) causeswasteful heat release. Therefore, according to this aspect of theinvention, the heating capacity of each utilization side heat exchanger(33 a) in heating operation can be enhanced and, in turn, the COP(coefficient of performance) of the refrigeration system can beincreased.

Furthermore, in the third aspect of the invention, the electricmotor-operated valve (34 b) once fully closed in performing the partialoperation is opened only for the second specified time t2 after thepassage of the first specified time t1. Therefore, according to thethird aspect of the invention, when the partial operation is continuedfor a long period of time, refrigerant liquefaction in each inactiveutilization side heat exchanger (33 b) can be certainly eliminated,which ensures the reliability of the refrigeration system.

Particularly, in the fourth aspect of the invention, during the partialoperation, the first specified time t1 and the second specified time t2are corrected based on the room temperature around each inactiveutilization side heat exchanger (33 b). Therefore, according to thefourth aspect of the invention, it can be certainly avoided that thefull-closure time of the electric motor-operated valve (34 b) becomeslonger than necessary to cause refrigerant liquefaction in theassociated inactive utilization side heat exchanger (33 b). Furthermore,according to the fourth aspect of the invention, it can be certainlyavoided that the open time of the electric motor-operated valve (34 b)becomes longer than necessary to cause wasteful heat release in theassociated inactive utilization side heat exchanger (33 b).

Furthermore, in the fifth aspect of the invention, the refrigerantdensity in each inactive utilization side heat exchanger (33 b) isdetected during the partial operation and when the refrigerant densityexceeds the specified refrigerant density, the fully closed electricmotor-operated valve (34 b) is temporarily opened. In other words, inthe fifth aspect of the invention, the amount of refrigerant accumulatedin each inactive utilization side heat exchanger (33 b) is indirectlydetermined and when the amount of refrigerant becomes large, theelectric motor-operated valve (34 b) is opened. Therefore, refrigerantliquefaction in each inactive utilization side heat exchanger (33 b) canbe certainly avoided.

Furthermore, according to the sixth aspect of the invention, by usingcarbon dioxide as refrigerant, the refrigeration system can operate in asupercritical cycle with natural refrigerant of relatively low criticaltemperature.

Furthermore, in the seventh aspect of the invention, the supply openingin each inactive utilization side heat exchanger (33 b) is closed by theopening/closing mechanism during the partial operation. Therefore, thedrop in the ambient temperature of the utilization side heat exchanger(33 b) can be restrained, whereby refrigerant liquefaction in theutilization side heat exchanger (33 b) can be further effectivelyavoided.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a piping diagram of a refrigerant circuit of an airconditioning system according to an embodiment.

[FIG. 2] FIG. 2 is a piping diagram showing the refrigerant flow of therefrigerant circuit during a full heating operation.

[FIG. 3] FIG. 3 is a piping diagram showing the refrigerant flow of therefrigerant circuit during a partial heating operation.

[FIG. 4] FIG. 4 is a P-H diagram (Mollier diagram) of a supercriticalcycle according to the above embodiment.

[FIG. 5] FIG. 5 is a P-H diagram (Mollier diagram) of a refrigerationcycle according to a conventional example.

[FIG. 6] FIG. 6 is a piping diagram showing the refrigerant flow of arefrigerant circuit during a partial heating operation of an airconditioning system according to a modification.

[FIG. 7] FIG. 7 is a graph showing behaviors of changes of refrigerantdensity and refrigerant temperature within the range from the entranceto the exit of an inactive indoor heat exchanger in the aboveembodiment.

[FIG. 8] FIG. 8 is a graph showing behaviors of changes of refrigerantdensity and refrigerant temperature within the range from the entranceto the exit of an inactive indoor heat exchanger in a conventionalexample.

LIST OF REFERENCE CHARACTERS

1 air conditioning system (refrigeration system)

10 refrigerant circuit

21 outdoor circuit (heat-source side circuit)

22 compressor

23 outdoor heat exchanger (heat-source side heat exchanger)

33 a first indoor heat exchanger (utilization side heat exchanger)

33 b second indoor heat exchanger (utilization side heat exchanger)

34 a first indoor expansion valve (electric motor-operated valve)

34 b second indoor expansion valve (electric motor-operated valve)

44 first room temperature sensor (room temperature sensor)

45 second room temperature sensor (room temperature sensor)

51 control means

52 correction means

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings.

A refrigeration system according to an embodiment constitutes aso-called multi-type air conditioning system (1) that can performheating and cooling of a room. As shown in FIG. 1, the air conditioningsystem (1) includes a single outdoor unit (20) placed outdoors and firstand second indoor units (30 a, 30 b) placed in different rooms.

The outdoor unit (20) is provided with an outdoor circuit (21)constituting a heat-source side circuit. The first indoor unit (30 a)and the second indoor unit (30 b) are provided with a first indoorcircuit (31 a) constituting a utilization side circuit and a secondindoor circuit (31 b) constituting another utilization side circuit,respectively.

The indoor circuits (31 a, 31 b) are connected in parallel via a firstconnection pipe (11) and a second connection pipe (12) to the outdoorcircuit (21). As a result, in this air conditioning system (1), arefrigerant circuit (10) operating in a refrigeration cycle bycirculating refrigerant therethrough is constituted. The refrigerantcircuit (10) is filled with carbon dioxide as refrigerant.

The outdoor circuit (21) is provided with a compressor (22), an outdoorheat exchanger (23), an outdoor expansion valve (24) and a four-wayselector valve (25). The compressor (22) is a fully-enclosed,high-pressure domed scroll compressor. The compressor (22) is suppliedthrough an inverter with electric power. In other words, the compressor(22) can be changed in capacity by changing the output frequency of theinverter and thereby changing the rotational speed of a motor for thecompressor. The outdoor heat exchanger (23) is a cross-fin-and-tube heatexchanger and constitutes a heat-source side heat exchanger. In theoutdoor heat exchanger (23), heat is exchanged between refrigerant andoutdoor air. The outdoor expansion valve (24) is composed of anelectronic expansion valve controllable in opening.

The four-way selector valve (25) has first to fourth ports. The four-wayselector valve (25) is connected at the first port to a discharge pipe(22 a) of the compressor (22), connected at the second port to theoutdoor heat exchanger (23), connected at the third port to a suctionpipe (22 b) of the compressor (22) and connected at the fourth port tothe first connection pipe (11). The four-way selector valve (25) isconfigured to be switchable between a position (the position shown inthe solid lines in FIG. 1) in which the first and fourth ports arecommunicated with each other and the second and third ports arecommunicated with each other and a position (the position shown in thebroken lines in FIG. 1) in which the first and second ports arecommunicated with each other and the third and fourth ports arecommunicated with each other.

The first indoor circuit (31 a) is provided with a first branch pipe (32a) connected at one end to the first connection pipe (11) and connectedat the other end to the second connection pipe (12). The first branchpipe (32 a) is provided with a first indoor heat exchanger (33 a) and afirst indoor expansion valve (34 a). The second indoor circuit (31 b) isprovided with a second branch pipe (32 b) connected at one end to thefirst connection pipe (11) and connected at the other end to the secondconnection pipe (12). The second branch pipe (32 b) is provided with asecond indoor heat exchanger (33 b) and a second indoor expansion valve(34 b).

Each of the indoor heat exchangers (33 a, 33 b) is a cross-fin-and-tubeheat exchanger and constitutes a utilization side heat exchanger. Ineach of the indoor heat exchangers (33 a, 33 b), heat is exchangedbetween refrigerant and room air.

The first indoor expansion valve (34 a) and the second indoor expansionvalve (34 b) are electric motor-operated valves and each constitutes anelectronic expansion valve controllable in opening. The first indoorexpansion valve (34 a) is provided in a part of the first branch pipe(32 a) close to the second connection pipe (12). The second indoorexpansion valve (34 b) is provided in a part of the second branch pipe(32 b) close to the second connection pipe (12). The first indoorexpansion valve (34 a) can control the flow rate of refrigerant flowingthrough the first indoor heat exchanger (33 a), while the second indoorexpansion valve (34 b) can control the flow rate of refrigerant flowingthrough the second indoor heat exchanger (33 b).

The refrigerant circuit (10) is further provided with a high-sidepressure sensor (40), a high-pressure temperature sensor (41), a firstrefrigerant temperature sensor (42) and a second refrigerant temperaturesensor (43). The high-side pressure sensor (40) detects the pressure ofrefrigerant discharged from the compressor (22). The high-pressuretemperature sensor (41) detects the temperature of refrigerantdischarged from the compressor (22). The first refrigerant temperaturesensor (42) is disposed at the exit of the first indoor heat exchanger(33 a) to detect the temperature of refrigerant just after flowing outof the first indoor heat exchanger (33 a). The second refrigeranttemperature sensor (43) is disposed at the exit of the second indoorheat exchanger (33 b) to detect the temperature of refrigerant justafter flowing out of the second indoor heat exchanger (33 b).

The first indoor unit (30 a) is provided also with a first roomtemperature sensor (44) in the vicinity of the first indoor heatexchanger (33 a). The first room temperature sensor (44) detects the airtemperature around the first indoor heat exchanger (33 a). The secondindoor unit (30 b) is provided also with a second room temperaturesensor (45) in the vicinity of the second indoor heat exchanger (33 b).The second room temperature sensor (45) detects the air temperaturearound the second indoor heat exchanger (33 b).

The refrigerant circuit (10) of the air conditioning system (1)according to this embodiment operates in a refrigeration cycle(supercritical cycle) in which the pressure of refrigerant dischargedfrom the compressor (22) is at or above the critical pressure.Furthermore, in the air conditioning system (1), each of the firstindoor unit (30 a) and the second indoor unit (30 b) is individuallyoperable. Specifically, the air conditioning system (1) can perform anoperation in which the first indoor unit (30 a) heats a room and thesecond indoor unit (30 b) is deactivated (hereinafter, referred to as apartial heating operation) or an operation in which both the firstindoor unit (30 a) and the second indoor unit (30 b) heat differentrooms (hereinafter, referred to as full heating operation).

The air conditioning system (1) is provided also with a controller (50)for controlling the openings of the indoor expansion valves (34 a, 34b). The controller (50) includes a control means (51) and a correctionmeans (52). The details of control of the controller (50) on theopenings of the indoor expansion valves (34 a, 34 b) will be describedlater.

Operational Behavior

Next, a description is given of the operational behavior of the airconditioning system (10) according to this embodiment. The airconditioning system (1) can perform an operation in which each indoorunit (30 a, 30 b) heats a room and an operation in which each indoorunit (30 a, 30 b) cools a room. A description is given below of theheating operation of the air conditioning system (1). In the heatingoperation, the four-way selector valve (25) is selected to the positionshown in FIGS. 2 and 3 so that the above-stated full heating operationand partial heating operation are selectively carried out.

<Full Heating Operation>

In the full heating operation, the first indoor expansion valve (34 a)and the second indoor expansion valve (34 b) are opened at apredetermined opening. As shown in FIG. 2, refrigerant condensed to thecritical pressure or higher by the compressor (22) flows through thefour-way selector valve (25) and the first connection pipe (11) and isthen distributed to the first branch pipe (32 a) and the second branchpipe (32 b).

The refrigerant having flowed into the first branch pipe (32 a) flowsthrough the first indoor heat exchanger (33 a). In the first indoor heatexchanger (33 a), the refrigerant releases heat to room air. In otherwords, the first indoor heat exchanger (33 a) performs a heatingoperation to heat room air, thereby heating the room in which the firstindoor unit (30 a) is installed. The refrigerant having flowed out ofthe first indoor heat exchanger (33 a) passes through the first indoorexpansion valve (34 a) and then flows into the second connection pipe(12).

On the other hand, the refrigerant having flowed into the second branchpipe (32 b) flows through the second indoor heat exchanger (33 b). Inthe second indoor heat exchanger (33 b), the refrigerant releases heatto room air. In other words, the second indoor heat exchanger (33 b)performs a heating operation to heat room air, thereby heating the roomin which the second indoor unit (30 b) is installed. The refrigeranthaving flowed out of the second indoor heat exchanger (33 b) passesthrough the second indoor expansion valve (34 b) and then flows into thesecond connection pipe (12).

The refrigerant combined in the second connection pipe (12) is reducedin pressure when passing through the outdoor expansion valve (24) andthen flows through the outdoor heat exchanger (23). In the outdoor heatexchanger (23), the refrigerant takes heat from outdoor air toevaporate. The refrigerant having flowed out of the outdoor heatexchanger (23) passes through the four-way selector valve (25) and isthen sucked into the compressor (22). In the compressor (22), therefrigerant is compressed to the critical pressure or higher.

<Partial Heating Operation>

In the partial heating operation, the air conditioning system (1)performs an operation in which the first indoor heat exchanger (33 a)performs the heating operation and, concurrently, the second indoor heatexchanger (33 b) halts the heating operation or an operation in whichthe second indoor heat exchanger (33 b) performs the heating operationand, concurrently, the first indoor heat exchanger (33 a) halts theheating operation. Here, a description is typically given of theoperation in which only the first indoor heat exchanger (33 a) performsthe heating operation with reference to FIG. 3.

In the partial heating operation, the control means (51) of thecontroller (50) opens the first indoor expansion valve (34 a) at apredetermined opening and sets the second indoor expansion valve (34 b)at a fully closed position. When the first indoor expansion valve (34 a)is opened, the first indoor heat exchanger (33 a) performs the heatingoperation as described previously. On the other hand, when the secondindoor expansion valve (34 b) is fully closed, refrigerant does not passthrough the second indoor expansion valve (34 b). Therefore, refrigerantdoes not flow through the second indoor heat exchanger (33 b), wherebythe second indoor heat exchanger (33 b) is made inactive.

When the second indoor heat exchanger (33 b) is thus made inactive,refrigerant gradually accumulates in the second indoor heat exchanger(33 b). However, also in the partial heating operation, the airconditioning system (1) of this embodiment, operates in a supercriticalcycle in which the pressure of refrigerant discharged from thecompressor (22) is at or above the critical pressure. Thus, even if theambient temperature of the second indoor heat exchanger (33 b) dropsowing to deactivation of the second indoor heat exchanger (33 b),refrigerant in the second indoor heat exchanger (33 b) does notcondense. Therefore, the rate of refrigerant liquefaction in the secondindoor heat exchanger (33 b) is significantly reduced as compared withthat in the case where an air conditioning system operates in asubcritical refrigeration cycle, for example, using HFC.

This point is described more closely with reference to FIGS. 4 and 5.FIG. 4 shows a P-H diagram of a supercritical cycle using carbon dioxidein this embodiment, and FIG. 5 shows a P-H diagram of a conventionalsubcritical refrigeration cycle using HFC.

In the conventional refrigeration cycle shown in FIG. 5, the pressure ofrefrigerant discharged from the compressor is below the criticalpressure. Specifically, for example, refrigerant after compressed in therefrigeration cycle has a pressure of 2.7 MPa, a temperature of 80° C.and a refrigerant density ρ₁ of 85 kg/m³. When the refrigerant condensesin the indoor heat exchanger, the refrigerant after condensation has apressure of 2.7 MPa, a temperature of 37° C. and a refrigerant densityρ₂ of 996 kg/m³. In other words, in the conventional refrigerationcycle, the density ratio (ρ₂/ρ₁) between refrigerant density ρ₂ at theexit of the indoor heat exchanger and refrigerant density ρ₁ at theentrance thereof is 11.72.

On the other hand, in this embodiment shown in FIG. 4, the pressure ofrefrigerant discharged from the compressor is above the criticalpressure. Specifically, for example, refrigerant after compressed inthis cycle has a pressure of 10 MPa, a temperature of 80° C. and arefrigerant density ρ₁ of 221 kg/M³. When the refrigerant releases heatin the indoor heat exchanger, the refrigerant after heat release has apressure of 10 MPa, a temperature of 35° C. and a refrigerant density ρ₂of 713 kg/m³. In other words, in a supercritical cycle according to thisembodiment, the density ratio (ρ₂/ρ₁) between refrigerant density ρ₂ atthe exit of the indoor heat exchanger and refrigerant density ρ₁ at theentrance thereof is 3.23.

As can be seen from the above, comparison of the density ratio (ρ₂/ρ₁)between before and after the indoor heat exchanger in the conventionalcycle with that in the refrigeration cycle according to this embodimentshows that the density ratio in the conventional cycle is three or moretimes greater than that in the refrigeration cycle according to thisembodiment. In other words, in the conventional refrigeration cycle,when refrigerant condenses in the inactive indoor heat exchanger, it hasa high density to reduce its volume and is therefore rapidly fed intothe inactive indoor heat exchanger. Thus, in the conventionalrefrigeration cycle, the rate of refrigerant liquefaction in theinactive indoor heat exchanger is relatively high.

In contrast, in this embodiment, even when refrigerant releases heat inthe inactive indoor heat exchanger, it has a relatively low density and,therefore, its volume is not so reduced. Thus, refrigerant is not so fedinto the indoor heat exchanger, whereby the rate of refrigerantliquefaction in the inactive indoor heat exchanger is relatively low.

However, when such a partial heating operation is continued for a longperiod of time, the amount of refrigerant liquefied in the second indoorheat exchanger (33 b) gradually increases. To cope with this, when afirst specified time t1 has passed since the start of the partialheating operation with full closure of the second indoor expansion valve(34 b), the control means (51) in this embodiment opens the secondindoor expansion valve (34 b) at a minute opening only for a secondspecified time t2. Thus, a minute flow rate of refrigerant flows throughthe second indoor heat exchanger (33 b) to increase the temperature ofthe second indoor heat exchanger (33 b) and the ambient temperaturethereof. As a result, refrigerant liquefaction in the second indoor heatexchanger (33 b) can be eliminated. Thereafter, when the secondspecified time t2 has passed, the control means (51) fully closes thesecond indoor expansion valve (34 b) again.

Furthermore, the amount of refrigerant liquefied in the second indoorheat exchanger (33 b) since the start of the partial heating operationwith full closure of the second indoor expansion valve (34 b) depends onthe ambient temperature of the second indoor heat exchanger (33 b). Inother words, if the temperature of a room where the second indoor heatexchanger (33 b) is installed is relatively low, the rate of refrigerantliquefaction in the second indoor heat exchanger (33 b) becomes high. Onthe other hand, if the temperature of the room is relatively high, therate of refrigerant liquefaction becomes low. To cope with this, thecorrection means (52) of the controller (50) in this embodiment controlsthe room temperature sensor (45) to detect the room temperature aroundthe inactive indoor heat exchanger (33 b) and corrects the above-statedfirst specified time t1 and second specified time t2 based on thedetected room temperature.

Specifically, if the room temperature detected by the second roomtemperature sensor (45) at the start of the partial heating operation isrelatively low, the correction means (52) decreases the first specifiedtime t1. Furthermore, if the room temperature detected by the secondroom temperature sensor (45) after the passage of the first specifiedtime t1 is relatively low, the correction means (52) increases thesecond specified time t2. As results of these corrections, the period oftime during which the second indoor expansion valve (34 b) is fullyclosed in the partial heating operation becomes short, wherebyrefrigerant liquefaction in the second indoor heat exchanger (33 b) canbe eliminated in advance. Either one of such corrections of the firstspecified time t1 and the second specified time t2 may be carried out orboth of them may be carried out.

On the other hand, if the room temperature detected by the second roomtemperature sensor (45) at the start of the partial heating operation isrelatively high, the correction means (52) increases the first specifiedtime t1. Furthermore, if the room temperature detected by the secondroom temperature sensor (45) after the passage of the first specifiedtime t1 is relatively high, the correction means (52) decreases thesecond specified time t2. As results of these corrections, the period oftime during which the second indoor expansion valve (34 b) is open inthe partial heating operation becomes short, whereby the inactive secondindoor heat exchanger (33 b) does not cause wasteful heat release.

Effects of Embodiment

In this embodiment, the air conditioning system (1), in which each of aplurality of indoor heat exchangers (33 a, 33 b) can individuallyperform a heating operation, operates in a supercritical cycle in whichthe pressure of refrigerant discharged from the compressor (22) is at orabove the critical pressure. Thus, even when the inactive indoorexpansion valve (34 b) is fully closed in the partial heating operation,refrigerant does not condense in the inactive indoor heat exchanger (33b). Therefore, according to this embodiment, the rate of refrigerantliquefaction in the inactive indoor heat exchanger (33 b) can besignificantly reduced. As a result, deficiency in refrigerant in theindoor heat exchanger (33 a) in heating operation can be avoided,thereby providing a sufficient heating capacity of the indoor heatexchanger (33 a) in heating operation.

Furthermore, in this embodiment, the indoor expansion valve (34 b) inthe deactivated unit is fully closed in performing the partial heatingoperation. Therefore, according to this embodiment, the inactive indoorheat exchanger (33 b) can be prevented from causing wasteful heatrelease. This increases the COP (coefficient of performance) of the airconditioning system (1).

Furthermore, in this embodiment, the indoor expansion valve (34 b) oncefully closed in performing the partial heating operation is opened onlyfor the second specified time t2 after the passage of the firstspecified time t1. Therefore, according to this embodiment, also whenthe partial heating operation is continued for a long period of time,refrigerant liquefaction in the inactive indoor heat exchanger (33 b)can be certainly eliminated, which certainly prevents shortage of amountof refrigerant in the indoor heat exchanger (33 a) in heating operation.

Furthermore, in this embodiment, during the partial heating operation,the first specified time t1 and the second specified time t2 arecorrected based on the room temperature around the inactive indoor heatexchanger (33 b). Therefore, according to this embodiment, it can beavoided that the full-closure time of the indoor expansion valve (34 b)becomes longer than necessary to cause refrigerant liquefaction in theinactive indoor heat exchanger (33 b). Furthermore, according to thisembodiment, it can be avoided that the open time of the indoor expansionvalve (34 b) becomes longer than necessary to cause wasteful heatrelease from refrigerant in the inactive indoor heat exchanger (33 b).This further increases the COP of the air conditioning system (1).

Modification of Control on Opening of Indoor Expansion Valve

In the above embodiment, after the indoor expansion valve (33 a, 33 b)in the deactivated unit is fully closed in the partial heatingoperation, this indoor expansion valve (34 b) is opened or closed basedon the first specified time t1 and the second specified time t2.However, instead of such control on the opening of the indoor expansionvalve (34 b), the opening of the indoor expansion valve (34 b) may becontrolled in a manner as shown in FIG. 6.

In a partial heating operation according to this modification, therefrigerant pressure detected by the high-side pressure sensor (40), therefrigerant temperature detected by the high-pressure temperature sensor(41), the refrigerant temperature detected by the first refrigeranttemperature sensor (42) and the refrigerant temperature detected by thesecond refrigerant temperature sensor (43) are output to the controller(50). Then, the controller (50) determines, based on the detected valuesof these sensors (40, 41, 42, 43), the density of refrigerant flowingthrough the inactive indoor heat exchanger (33 b) during the partialheating operation. In other words, each of the sensors (40, 41, 42, 43)constitutes a refrigerant density detecting device for detecting therefrigerant density in the inactive indoor heat exchanger (33 b).

Specifically, for example, in performing the same partial heatingoperation as in the above embodiment, the control means (51) firstbrings the opening of the second indoor expansion valve (34 b) into afully closed position. When the partial heating operation is continuedfor a long period of time, refrigerant gradually liquefies in the secondindoor heat exchanger (33 b).

To cope with this, the control means (51) in this modificationdetermines the refrigerant density in the inactive second indoor heatexchanger (33 b) from the refrigerant pressure and the refrigeranttemperature. Specifically, for example, in the case where the secondindoor heat exchanger (33 b) is made inactive, the controller (50)determines the refrigerant density in the second indoor heat exchanger(33 b) based on the refrigerant pressure detected by the high-sidepressure sensor (40), the refrigerant temperature detected by thehigh-pressure temperature sensor (41) and the refrigerant temperaturedetected by the second refrigerant temperature sensor (43) in thedeactivated unit. In fact, the refrigerant pressure detected by thehigh-side pressure sensor (40) is substantially equal to the refrigerantpressure in the second indoor heat exchanger (33 b). Furthermore, therefrigerant temperature detected by the high-pressure temperature sensor(41) can be considered as the temperature of refrigerant flowing intothe second indoor heat exchanger (33 b) and the refrigerant temperaturedetected by the second refrigerant temperature sensor (43) can be thetemperature of refrigerant having flowed out of the second indoor heatexchanger (33 b). Therefore, from these temperatures of inflowrefrigerant and outflow refrigerant, the average temperature ofrefrigerant in the indoor heat exchanger (33 b) can be determined. Then,from this average refrigerant temperature and the above refrigerantpressure, the average refrigerant density of refrigerant in the secondindoor heat exchanger (33 b) can be determined.

The refrigerant density thus obtained gives an indication of the amountof refrigerant accumulated in the second indoor heat exchanger (33 b).Then, when the refrigerant density obtained from the detected values ofthe sensors (40, 41, 43) exceeds a specified refrigerant density afterthe start of the partial heating operation with full closure of thesecond indoor expansion valve (34 b), the control means (51) in thismodification determines that a large amount of refrigerant isaccumulated in the second indoor heat exchanger (33 b), and temporarilyopens the second indoor expansion valve (34 b). As a result, refrigerantliquefaction in the second indoor heat exchanger (33 b) can be certainlyeliminated.

On the other hand, in a partial heating operation in which the firstindoor heat exchanger (33 a) is made inactive and the second indoor heatexchanger (33 b) performs a heating operation, the refrigerant densityin the first indoor heat exchanger (33 a) is determined based on thedetected values of the high-side pressure sensor (40), the high-pressuretemperature sensor (41) and the first refrigerant temperature sensor(42) in the deactivated unit. In this case, when the refrigerant densityexceeds the specified refrigerant density, the first indoor expansionvalve (34 a) is opened to eliminate refrigerant liquefaction in thefirst indoor heat exchanger (33 a).

Effects of Modification

In this modification, the refrigerant density in the inactive indoorheat exchanger (33 b) is detected during the partial heating operationand when the refrigerant density exceeds the specified refrigerantdensity, the fully closed indoor expansion valve (34 b) is temporarilyopened. In other words, in this modification, the amount of refrigerantaccumulated in the inactive indoor heat exchanger (33 b) is indirectlydetermined and when the amount of refrigerant becomes large, the indoorexpansion valve (34 b) is opened. Therefore, refrigerant liquefaction inthe inactive indoor heat exchanger (33 b) can be certainly avoided.

Furthermore, also in this modification, the refrigerant circuit (10)operates in a supercritical cycle during the partial heating operation,whereby the rate of refrigerant liquefaction in inactive one of theindoor heat exchangers (33 a, 33 b) can be significantly reduced.

Furthermore, when the refrigerant circuit (10) operates in asupercritical cycle in the above manner, the average refrigerant densityin the inactive indoor heat exchanger (33 b) can be more accuratelyobtained. Specifically, with reference to changes of refrigerant density(or refrigerant temperature) from the entrance to the exit of aninactive indoor heat exchanger in a conventional example (an airconditioning system in which the refrigerant circuit operates in arefrigeration cycle in which high-side pressure is a subcriticalpressure) as for example shown in FIG. 8, it can be noted that thebehavior of the changes has poor linearity. The reason for this is thatin the conventional example refrigerant in the inactive indoor heatexchanger condenses to change its phase. Therefore, in order toaccurately obtain the amount of refrigerant accumulated in the indoorheat exchanger, it is necessary to detect the refrigerant density (orrefrigerant temperature) at a plurality of points (for example, three ormore points). This increases the number of temperature sensors.

In contrast, with reference to changes of refrigerant density (orrefrigerant temperature) in the inactive indoor heat exchanger (33 b) inthis embodiment as shown in FIG. 7, it can be noted that the behavior ofthe changes has a relatively high linearity. The reason for this is thatin this embodiment refrigerant of critical pressure or higher pressureaccumulates in the indoor heat exchanger (33 b) and, therefore, therefrigerant in the indoor heat exchanger (33 b) does not change itsphase from the entrance to the exit. Therefore, according to thisembodiment, by determining the refrigerant densities at the entrance andthe exit in the manner shown in the above modification, the behavior ofrefrigerant densities from the entrance to the exit of the indoor heatexchanger (33 b) can be accurately predicted based on a data tablepreviously stored in the controller (50) (such as data on behavior ofchanges of refrigerant density or behavior of changes of refrigeranttemperature). Then, by determining the timing of opening of the indoorexpansion valve (34 a, 34 b) based on the refrigerant density thusobtained, refrigerant liquefaction in the inactive indoor heat exchanger(33 b) can be more certainly avoided.

<<Other Embodiments>>

In the air conditioning system (1) according to the above embodiment,each of the supply openings, through which air having passed through theutilization side heat exchangers (33 a, 33 b) is supplied, may beprovided with an opening/closing mechanism, such as a louver, that canopen and close the supply opening. Furthermore, during the partialheating operation as described above, only the supply opening associatedwith the inactive utilization side heat exchanger (33 b) may be closedby the opening/closing mechanism. In this case, it can be prevented thatheat of refrigerant accumulated in the inactive utilization side heatexchanger (33 b) escapes through the supply opening to the room space.Therefore, the drop in the ambient temperature of the utilization sideheat exchanger (33 b) can be restrained, whereby refrigerantliquefaction in the utilization side heat exchanger (33 b) can befurther effectively avoided. If a sealing material, such as packing, isprovided around the opening/closing mechanism, such as a louver, this ispreferable because the sealing property of the supply opening whensealed is enhanced.

The above embodiments are merely preferred embodiments in nature and arenot intended to limit the scope, applications and use of the invention.

INDUSTRIAL APPLICABILITY

As can be seen from the above description, the present invention isuseful as measures against refrigerant liquefaction in inactive ones ofutilization side heat exchangers in a refrigeration system in which eachof a plurality of utilization side heat exchangers can individuallyperform a heating operation.

1. A refrigeration system comprising a refrigerant circuit formed sothat a plurality of utilization side circuits including their respectiveutilization side heat exchangers and electric motor-operated valvesassociated with the respective utilization side heat exchangers areconnected in parallel to a heat-source side circuit including acompressor and a heat-source side heat exchanger, each of theutilization side heat exchangers being capable of individuallyperforming a heating operation to release heat from refrigerant in theutilization side heat exchanger, wherein the refrigerant circuit isconfigured to operate in a refrigeration cycle in which the pressure ofrefrigerant discharged from the compressor is at or above the criticalpressure.
 2. The refrigeration system of claim 1, further comprising acontrol means that, in performing an operation in which at least onesaid utilization side heat exchanger in heating operation and at leastone said inactive utilization side heat exchanger coexist, fully closesthe electric motor-operated valve associated with the at least oneinactive utilization side heat exchanger.
 3. The refrigeration system ofclaim 2, wherein when a first specified time t1 has passed since fullclosure of the electric motor-operated valve associated with the atleast one inactive utilization side heat exchanger, the control meanstemporarily opens the electric motor-operated valve for a secondspecified time t2.
 4. The refrigeration system of claim 3, wherein eachof the utilization side heat exchangers is placed in a room andconfigured to release heat from refrigerant to a room air, roomtemperature sensors for detecting the temperatures of rooms associatedwith the respective utilization side heat exchangers are provided aroundthe respective utilization side heat exchangers, and the refrigerationsystem further comprises a correction means that corrects one or both ofthe first specified time t1 and the second specified time t2 based onthe temperature detected by the room temperature sensor associated withthe at least one inactive utilization side heat exchanger.
 5. Therefrigeration system of claim 2, further comprising refrigerant densitydetecting devices for detecting the refrigerant densities in theassociated utilization side heat exchangers, wherein when therefrigerant density detected by at least one said refrigerant densitydetecting device associated with the at least one inactive utilizationside heat exchanger exceeds a specified refrigerant density after fullclosure of the electric motor-operated valve associated with the atleast one inactive utilization side heat exchanger, the control meanstemporarily opens the electric motor-operated valve.
 6. Therefrigeration system of any one of claims 1 to 5, the refrigerantcircuit is filled with carbon dioxide as refrigerant.
 7. Therefrigeration system of any one of claims 2 to 5, further comprisingsupply openings through which air having passed through the associatedutilization side heat exchangers is let out and opening/closingmechanisms for opening and closing the associated supply openings,wherein each of the opening/closing mechanisms is configured to open thesupply opening of the associated utilization side heat exchanger when inheating operation and close the supply opening of the associatedutilization side heat exchanger when inactive.